This invention relates generally to a system for acoustically transmitting data along a drill string, and more particularly to an analog circuit for controlling acoustic transducer arrays used for transmitting and receiving data signals through a drill string.
Deep wells of the type commonly used for petroleum or geothermal exploration are typically less than 30 cm (12 inches) in diameter and on the order of 2 km (1.5 miles) long. These wells are drilled using drill strings assembled from relatively light sections (either 30 or 45 feet long) of drill pipe that are connected end-to-end by tool joints, additional sections being added to the uphole end as the hole deepens. The downhole end of the drill string typically includes a drill collar, a dead weight assembled from sections of relatively heavy lengths of uniform diameter collar pipe having an overall length on the order of 300 meters (1000 feet). A drill bit is attached to the downhole end of the drill collar, the weight of the collar causing the bit to bite into the earth as the drill string is rotated from the surface. Sometimes, downhole mud motors or turbines are used to turn the bit. Drilling mud or air is pumped from the surface to the drill bit through an axial hole in the drill string. This fluid removes the cuttings from the hole, provides hydrostatic head which controls the formation gases, and sometimes provides cooling for the bit.
Communication between downhole sensors of parameters such as pressure or temperature and the surface has long been desirable. Various methods that have been tried for this communication include electromagnetic radiation through the ground formation, electrical transmission through an insulated conductor, pressure pulse propagation through the drilling mud, and acoustic wave propagation through the metal drill string. Each of these methods has disadvantages associated with signal attenuation, ambient noise, high temperatures, and compatibility with standard drilling procedures. The most commercially successful of these methods has been the transmission of information by pressure pulse in the drilling mud (known as mud pulse telemetry). However, attenuation mechanisms in the mud limit the transmission rate.
Faster data transmission may be obtained by the use of acoustic wave propagation through the drillstring. While this method of data transmission has heretofore been regarded as impractical, a significantly improved method and apparatus for the acoustic transmission of data through a drillstring is disclosed in U.S. patent application Ser. No. 605,255 filed Oct. 29, 1990, entitled "Acoustic Data Transmission Through a Drillstring" and invented by Douglas Drumheller, (all of the contents of which are fully incorporated herein by reference). The method and apparatus disclosed in this patent application will permit large scale commercial use of acoustic telemetry in the drilling of deep wells for petroleum and geothermal exploration.
U.S. Ser. No. 605,255 describes an acoustic transmission system which employs a transmitter for converting an electrical input signal into acoustic energy within the drill collar. The transmitter includes a pair of spaced transducers which are controlled by a digital circuit. This digital circuit controls phasing of electrical signals to and from the transducers so as to produce an acoustical signal which travels in only one direction. While suitable for its intended purpose, there is a need for improved, less complicated circuitry for use in controlling both acoustic transmitters and receivers in acoustic telemetry systems of the type described in U.S. Ser. No. 605,255.